The present invention relates to a method and apparatus for making welded large pipes, in particular helical-seam pipes, from hot strip of steel.
Welded large pipes involved here include longitudinally welded pipes or helical-seam pipes, whereby helical-seam pipes are made at a diameter range of about 400 mm to 3050 mm and at wall thicknesses of 5 mm to 30 mm, and involve pipes which are continuously shaped into helical pipes with constant radius of curvature from a hot strip or sheet metal in a shaping device, whereby converging edges of the strip are welded together. Unlike a manufacture of longitudinally welded pipes which require a particular sheet metal width for each pipe diameter, helical-seam pipes can be made with different pipe diameter from a strip or sheet metal width. The infeed angle of the strip into a shaping unit can hereby be changed whereby a decrease in the infeed angle (at constant strip width) is accompanied by an increase of the pipe diameter.
Projects involving large pipelines, in particular water mains, will be increasingly located in regions that have a high growth in population. Oftentimes, these regions are characterized by a shortness of water and underdeveloped infrastructure. As a result, transport of pipes and logistics becomes time-consuming and costly. In addition, the economy of countries in such regions is frequently precarious and foreign currencies are scarce so that attempts are underway to avoid import of goods and to increase own efforts to satisfy the demand. In order to produce helical-seam pipes, these countries are therefore increasingly involved in the establishment of domestic production sites that can be quickly set up and operated. These types of plants are typically devised to meet a single purpose.
The publication Stahlrohr Handbuch (Steel Pipe Handbook) published by Vulkan-Verlag, Essen, Germany, 12th edition 1995, pp. 143–152, describes a method and apparatus for making welded helical-seam pipes, with the apparatus including a stationary unit for producing submerged-arc welded helical-seam pipes. The stationary unit includes a strip shaper, a submerged-arc welder, an ultrasonic inspection of the welded seam and base material before and after welding operation, a cutter for trimming the pipe strand into single pipe sections, and a finishing unit.
Production of a helical-seam pipe involves initially unwinding of a hot strip from a coil and welding of the hot strip, after suitable preparation, to the trailing end of a previously payed-out hot strip to thereby realize a quasi endless strip and thus a continuous pipe welding operation. The strip passes through leveling rollers to level the strip commensurate with the demands on strip flatness. The edges of the strip are trimmed, i.e. the strip is cut to a constant width. At the same time, further tools, such as milling cutter or shearing blades, prepare the edges of the strip to make it fit for subsequent welding operation. Before being shaped into a pipe, the edges are bent to prevent a roof formation in the area of the welded seam.
After shaping into a pipe configuration, submerged-arc welding is applied to connect the converging strip edges on the inside in about 6 o'clock position and, half a turn further, on the outside in 12 o'clock position. As an alternative, pipe shaping may also be combined with tack welding, whereby the tacked pipe is then welded on separate welding stands on the inside and the outside.
Immediately thereafter, the welded seam is inspected by ultrasound for possible flaws, and the pipe strand is cut by a conjointly operating pipe cutter into single pipe sections which are then transferred to a finishing unit to subject the welded helical-seam pipe for visual control to evaluate the pipe condition after production before advance to further processing operations, such as optional finishing works, taking of specimen for destructive inspection, pipe end finishing operation, water pressure sample of the finished pipe, ultrasonic control of the entire welded seam, and X-ray screening of the welded seam if needed.
The process described in Stahlrohr Handbuch requires many inspection and control steps during production of submerged-arc welded helical-seam pipes or longitudinally welded pipe to meet the required quality for the end product. This process is thus time-consuming and unsuitable to satisfy demands for current welding techniques and continuous process control. As this process is intended for a long-term stationary operation, its application is also unsuitable for single-purpose plants which require a quick set up and which can be quickly put into operation.
In summary, the process described in the Stahlrohr Handbuch has the following shortcomings:                very long manufacturing chain with many inspection and control steps, resulting in a very outsized and cost-intensive structure;        transfer of the plant from site to site (mobility) is cost-intensive and time-consuming;        submerged-arc welding results in a relatively large-volume melt bath that makes the use of modern systems for quality assurance difficult;        submerged-arc welding normally requires Y joint or double-Y-joint as edge preparation, accompanied by an increase in welding additives (welding wire and powder) in dependence of the sheet thickness and resulting in larger melt baths, higher risk for flaws, and welding costs;        the size of the melt bath adversely affects, depending on the pipe diameter (curvature of the pipe wall), an increase of the welding speed. This is especially true when thin-walled pipes are involved;        submerged-arc welding results in a significant weld camber at the inner seam and outer seam, thereby adversely affecting a later coating of the inside and outside of the pipe. The weld camber, on one hand, results also in increased wear of the coating material, and, on the other hand, there is a risk of damage to the outer pipe coating in the area of the weld camber during transport and handling.        
German Pat. No. DE 197 58 001 C2 describes a method and apparatus for making helical-seam pipes through laser welding, whereby laser welding is applied to configure the seam substantially flat, to provide a fine-grained structure and strength properties of the seam to at least correspond to the strength properties of the base material, and to provide helical-seam pipes in the form of continuous cylindrical pipes. This method is unsuitable to produce a machine-ready high quality pipe by way of a mobile plant design.
It would therefore be desirable and advantageous to provide an improved method and apparatus for making welded large pipes, in particular helical-seam pipes, to obviate prior art shortcomings.